A field effect transistor (FET) is a transistor that relies on an electric field to control conductivity of a channel in a semiconductor material. When the channel is an active channel, majority charge carriers, electrons or holes, flow through the channel from a source of the FET to a drain of the FET. Conductivity of the channel is a function of potential applied between a gate of the FET and the source. In this regard, in an enhancement-mode only FET, when a voltage between the gate and the source exceeds a threshold voltage of the FET, a low-resistance channel is established, such that the majority charge carriers can flow from the drain to the source. Conversely, when the voltage between the gate and the source drops below the threshold voltage of the FET, a high-resistance channel is established, such that the majority charge carrier flow is impeded.
When the FET is used as an electronic switch, the FET has either an ON state, in which current can flow between the source and the drain; or an OFF state, in which current is impeded from flowing between the source and the drain. As such, the FET may operate in the ON state when the voltage between the gate and the source exceeds the threshold voltage of the FET. Conversely, the FET may operate in the OFF state when the voltage between the gate and the source is below the threshold voltage of the FET. Therefore, if the source of the FET is coupled to ground, a voltage swing of a control signal feeding the gate of the FET must exceed the threshold voltage to ensure proper selection between the ON state and the OFF state.
A junction FET (JFET) includes a P-N junction between the gate of the JFET and the channel of the JFET. Normally, JFETs are depletion mode only devices to prevent forward current flow though the P-N junction of the JFET. A metal oxide semiconductor FET (MOSFET) includes an oxide layer between a metal gate of the MOSFET and the channel of the MOSFET to insulate the gate from the channel. It should be noted that the term MOSFET is also commonly used to describe FETs having an oxide layer between a semiconductor gate, instead of a metal gate, of the MOSFET and the channel of the MOSFET to insulate the gate from the channel. The semiconductor gate may include polysilicon. In the present disclosure, the term MOSFET includes any FET with an oxide layer between the gate and the channel. MOSFETs may be enhancement mode only devices, depletion mode only devices, or enhancement mode-depletion mode devices. An N-type FET has a source and a drain with N-type semiconductor material, and a P-type FET has a source and a drain with P-type semiconductor material.
MOSFETs may be used as electronic switches to construct logic circuits, which are commonly used in digital systems. Such logic circuits normally provide output voltage swings that are compatible with the threshold voltages of the MOSFETs used in the logic circuits. However, in some digital systems, certain MOSFETs may be used for special applications, such as high speed, high voltage, high temperature, high current, or the like. Such MOSFETs may have lower transconductance and/or higher threshold voltages than other MOSFETs in the digital system, thereby creating a voltage swing and desired gate voltage incompatibility. As such, there is need for an interface circuit, which receives an input signal having a standard voltage swing and provides an output signal having a larger voltage swing that may be used to properly drive a high gate drive voltage MOSFET.